# A whole whack of question (audio related)

Discussion in 'Audio' started by luke skywalker, Jun 11, 2012.

1. ### luke skywalker

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Jun 11, 2012
So I'm going to build my car a stereo from scratch. I have lots of experience building amplifiers and such, finding AV based on the load. My biggest void of knowledge at this point is the speakers, the way I look at it I'll have my aux stereo input w/ 3 pins, left right and ground. I'll have two amplifier circuits using BJT's or mosfets, at the output of the first amp I'll put the front left and back left in parallel and the output at the second amp I'll put the front right and back right in parallel.

First off, should there be a common ground? ie. Should the aux ground share the battery ground share the amplifier ground? Knowing from school I feel there should but I would like confirmation.

Second off, what makes a speaker make noise, I know this sounds like a broad question but basically I know that the diaphragm will resonate when an AC voltage is applied to it. But what determined the decibels? Current, Voltage or Power? I know the speaker will have a max power rating, does it have a max voltage rating? max current rating? should I fuse the output? is there a mathematical way to figure out the decibels those guys will create with the power dissipated in the speaker?

Also this isn't really electronically related but does anyone know where to get the speaker specs for my car? Its a Jeep Grand Cherokee 96'

Thanks, much appreciated

2. ### Harald KappModeratorModerator

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Nov 17, 2011
Unless you have separate batteries for the amplifiers plus differential signalling (optical signal transmission) you will definitely have a common ground.
I doubt you have that (at least you don't describe anything along these lines). And if you can't guarantee separate grounds, it is better to use a defined common ground than to leave the grounding to chance.

Therefore the question is how to wire his common ground.

One general advice is: avoid ground loops. That is, avoid connection schemes where a current has more than one ground path to return to the battery. In your setup you have to take into account that the stereo source comes from a 3 pin terminal which already has a common ground. you will have to split this 3-pin terminal into 2*2-pin terminals, one for each amplifier.
So you should connect ground of the two separate amplifiers by a very low impedance connection (short, thick wire, preferably a copper strap). Otherwise you risk that some of the ground current takes the path over the ground wire of the input line which will lead to crosstalk (at best).

Any of the above since P= U*I plus I=U/Z where Z is the impedance of the speaker, so P=U²/Z or P=I²*Z.
Thus knowing the characteristic impedance Z of the speaker any one of P, U or I suffices to compute the others.

Physically it is the current that drives a normal magnetodynamic speaker. The current generates a magnetic field which moves a coil within the fixed magentic field of the speaker. The coil in turn moves the membrane and generates the sound.
There are also electrostatic speakers which use the voltage as the driving force.

Harald

3. ### Civilian

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Jun 12, 2012
I'm an audio expert, but I'm not a car audio expert so I may misunderstand what you're referring to here. It sounds like you are talking about your line signal either to or from the car deck. In which case, the third wire isn't a ground, it's a common. Not a
'common ground', but a 'common conductor' for the line signal.

I could spend a day answering that paragraph. But it sounds like you are interested in knowing what the decibel output of your stereo will be. Every speaker has a decibel rating at a specified distance, at it's max current, and usually at a specified frequency. ie; 110db @ 1 meter @ 1k

The output level of a speaker isn't set by only how much current you send to it, although that is a big factor. Just because a speaker has a higher power rating doesn't mean it will be louder or sound better. It's also set by how efficient the design of the speaker is.

Also keep in mind with decibel ratings:
-when the distance doubles the volume drops 6db. Which is halving the volume (debatable).
-You will get a 3db gain when you double the watts to a speaker.

4. ### Harald KappModeratorModerator

11,416
2,619
Nov 17, 2011
Basically right, but the amplifiers I know use common ground for that purpose.
Even if that were not the case (e.g. isolated output) this common conductor will short-circuit ground at the input of the two receivers - unless the inputs are isolated, too. But as far as I know isolated inputs and outputs were common in the days of tube amplifiers to match impedances but are alomost completely gone today.

Harald